Key differentiators in my experience, primarily from a business perspective, but a lot applies to consumers as well:
- Fedex tends to leave fewer "high dollar" packages exposed at a delivery address, and require more signatures in general.
- Fedex has saturday delivery that actually works.
- UPS is preferable for larger/more sturdy items that can be shipped ground (no hurry). Fedex ground service is a terrible afterthought to their superlative express offerings.
- Due to UPS being unionized, it's much harder to deal with a situation in which you have issues with your assigned delivery/pickup person.
In short: if it's not going to break, is huge, and/or you're not in a rush, UPS. Everything else, Fedex. Most of the time, pricing will also favor the companies in parallel to this rule.